CBSE Class 12-science Answered

For cubic closed packed structure or FCC:

Contribution by atoms on the corners = 1/8 x 8 = 1 atom

Contribution by atoms on the faces = ½ x 6 = 3

Therefore, number of atoms present in the unit cell = 1+3 = 4 atoms

For hexagonal closest packed structure:

Contribution by atoms of the hex shape at top = 1/6 x 6 = 1 atom

Contribution by atoms of the hex shape at bottom = 1/6 x 6 = 1 atom

Contribution by the one atom at the centre of top and bottom hex each = 1

Contribution by the three atoms in the center = 3 atoms

Therefore, number of atoms per unit cell = 1+1+1+3 = 6 atoms